Advance input pointer when LZ4 compressing data

LZ4File_write() did not advance the input pointer on subsequent invocations of
LZ4F_compressUpdate(). As a result the generated compressed output would be a
compressed version of the same input chunk.

Tests failed to catch this error because the data would comfortably fit
within the default buffer size, as a single chunk. Tests have been added
to provide adequate coverage of multi-chunk compression.

WriteDataToArchiveLZ4() which is also using LZ4F_compressUpdate() did
not suffer from this omission.

Author: Georgios Kokolatos <gkokolatos@pm.me>
Reported-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/ZFhCyn4Gm2eu60rB%40paquier.xyz

src/bin/pg_dump/compress_io.h

@@ -17,7 +17,13 @@
 
 #include "pg_backup_archiver.h"
 
-/* Default size used for IO buffers */
+/*
+ * Default size used for IO buffers
+ *
+ * When changing this value, it's necessary to check the relevant test cases
+ * still exercise all the branches. This applies especially if the value is
+ * increased, in which case the overflow buffer may not be needed.
+ */
 #define DEFAULT_IO_BUFFER_SIZE	4096
 
 extern char *supports_compression(const pg_compress_specification compression_spec);

src/bin/pg_dump/compress_lz4.c

@@ -588,6 +588,8 @@ LZ4Stream_write(const void *ptr, size_t size, CompressFileHandle *CFH)
 			errno = (errno) ? errno : ENOSPC;
 			return false;
 		}
+
+		ptr = ((const char *) ptr) + chunk;
 	}
 
 	return true;

src/bin/pg_dump/t/002_pg_dump.pl

@@ -3108,6 +3108,52 @@ my %tests = (
 		},
 	},
 
+	'CREATE TABLE test_compression_method' => {
+		create_order => 110,
+		create_sql   => 'CREATE TABLE dump_test.test_compression_method (
+						   col1 text
+					   );',
+		regexp => qr/^
+			\QCREATE TABLE dump_test.test_compression_method (\E\n
+			\s+\Qcol1 text\E\n
+			\Q);\E
+			/xm,
+		like => {
+			%full_runs,
+			%dump_test_schema_runs,
+			section_pre_data     => 1,
+		},
+		unlike => {
+			exclude_dump_test_schema => 1,
+			only_dump_measurement    => 1,
+		},
+	},
+
+	# Insert enough data to surpass DEFAULT_IO_BUFFER_SIZE during
+	# (de)compression operations
+	'COPY test_compression_method' => {
+		create_order => 111,
+		create_sql   => 'INSERT INTO dump_test.test_compression_method (col1) '
+		  . 'SELECT string_agg(a::text, \'\') FROM generate_series(1,4096) a;',
+		regexp => qr/^
+			\QCOPY dump_test.test_compression_method (col1) FROM stdin;\E
+			\n(?:\d{15277}\n){1}\\\.\n
+			/xm,
+		like => {
+			%full_runs,
+			data_only                => 1,
+			section_data             => 1,
+			only_dump_test_schema    => 1,
+			test_schema_plus_large_objects    => 1,
+		},
+		unlike => {
+			binary_upgrade           => 1,
+			exclude_dump_test_schema => 1,
+			schema_only              => 1,
+			only_dump_measurement    => 1,
+		},
+	},
+
 	'CREATE TABLE fk_reference_test_table' => {
 		create_order => 21,
 		create_sql   => 'CREATE TABLE dump_test.fk_reference_test_table (